More than you’ll ever want to know, but here goes.
It’s “diapers” or “diaper” depending on whether you’re talking about one diaper or two or more diapers. Change his diaper, and diaper rash v. change their diapers. The word “diaper” originally referred to the type of cloth rather than its use. It’s was originally a white linen cloth with a diamond pattern interspersed. Diapering is also used in decorative art generally for an all-over repeating pattern. The British and Canadians use napkin or nappy rather than diaper. Just as you wouldn’t say change his napkins, you wouldn’t say change his diapers. The first known reference is in Shakespeare’s The Taming of the Shrew: “Another bear the ewer, the third a diaper.” Another example is in R.H. Barnum, The Jackdaw of Rheims, “Thomas Ingoldsby” (1837): “One little boy more, A napkin bore, Of the best white diaper, fringed with pink, And a Cardinal’s Hat mark’d in ‘permanent ink.'”
As for “pants”…circa 800 a.d., the fool in a comic production was called Panteleon, named after a Christian doctor was condemned to death by the Romans in the 3rd Century for aiding the poor. The doctor was canonized by the Church after surviving six executions giving him the name Saint Pantaleon. Pan is Greek for “all” and Leo is Latin for “lion,” thus “All Lion.” This name was then adopted by lots of people over the years for boys’ first names, including the fool in the production. The actor was dressed in breeches that were tight below the knee but which bloused out in a full puffy fashion from the waist to the knee. In the 18th century the costume became one worn by many men. A famous portrait found in the Louvre shows Louis XIV in a full pose, showing off his legs in a ”pantaloon” costume. The term was shortened to “pants” in the 1840s. Thus it’s not singular nor plural. Bad argument by the Frontburnervian.
The word “underwears” doesn’t exist. It’s not in any dictionary, so anyone who says it does is wrong. It’s underwear or underpants. It’s name, not surprisingly, comes from the fact that it’s clothing worn under other clothing, including pants.